Optimization Of Boron Isotope Analysis Method For Carbonate In Salt Lake And Its Application In The East Taijinar Salt Lake Section

The separation and purification methods of boron and the determination technology of boron isotope are the basis of boron isotope application research.At the same time,the optimization of boron isotope analysis methods can promote the rapid development of boron isotope geochemistry.Based on previous studies,this dissertation mainly involves the extraction of boron from carbonate sediments of salt lakes,the elimination of precipitation in pretreatment process,the elimination of the influence of organic matter on the determination of boron isotope,the analysis of boron isotope fractionation characteristics of inorganic carbonate deposits in artificial magnesium free brine under pH control and carbonate boron isotope characteristics in East Taijinar salt lake section.The research results are as follows:1.Different solvents?Including different concentrations of HCl,HAc and EDTA-2Na?were used to extract boron from carbonate components in sediments.The results show that 1 mol/L HAc is the best solvent for extracting boron from carbonate components of sediments.Compared with other solvents,1 mol/L HAc has little effect on clay and gypsum,and can effectively dissolve carbonate minerals such as calcite and dolomite.At the same time,the amount of sub boiling ammonia water used to adjust the pH of sample solution to weak alkali before treating the sample with resin is less,so the background introduced is also less.2.It was found that EDTA-2Na can combine with metal ions to form a stable complex.So adding a proper amount of saturated EDTA-2Na solution to the acid solution can effectively avoid the precipitation of hydroxide in the acid solution under alkaline conditions.The influence of EDTA-2Na residue can be inhibited effectively by washing the resin bed with ammonia and continuously ablating the sample with increasing current.After the entire process,the recovery rate of boron is more than95%,and the boron isotopic ratio of NIST SRM 951 range from 4.05146?4.05628.The boron isotopic composition(?¹¹B)of nature sediments of Sanya,Dachaidan Lake core and Dachaidan Lake Core intergranular brine are+17.64‰±0.70‰,-5.74‰±0.14‰and+1.8‰±0.01‰.3.In this paper,the combination of ion exchange and micro sublimation technology was used to remove the interference of EDTA-2Na in the process of boron isotope determination.First,Amberlite IRA 743 resin was used to separate and purification boron and then micro sublimation was used to remove the residual of EDTA-2Na.Finally,the mixed resin was used to remove the residual impurities.The results show that the recovery of boron can reach more than 95%,and the range of¹¹B/¹⁰B ratio of NIST SRM 951 is 4.04984±0.00019?4.05124±0.00017.There is no fractionation of boron in NIST SRM 951.The?¹¹B values of natural samples are close to those of three-step ion exchange method and multiple precipitation method.The pretreatment process of the combined method is simple and efficient.It provides a good choice for accurate analysis of boron isotope of samples with low boron content.4.The experiments of carbonate deposition in artificial magnesium free brine under different pH values were carried out.The experimental results show that in the absence of the formation of Mg?OH?₂,the boron content increase with the increase of pH of the mother solution.This indicates that pH has a great influence on the content of boron carbonate.On the one hand,the high pH value increases the content of B?OH?₄^-and causes the co-precipitation of calcium carbonate and boron,resulting in the increase of boron content in calcium carbonate.On the other hand,the high content of Ca²⁺in the brine accelerates the precipitation of calcium carbonate,which leads to the increase of boron incorporation into calcium carbonate.The results of?¹¹B in sedimentary calcium carbonate show that the fractionation coefficient between calcium carbonate and mother liquor increases with the increase of pH value of mother liquor,which indicates that calcium carbonate is relatively rich in ¹¹B.With the increase of pH value,the isotopic fractionation degree of boron between calcium carbonate and mother liquor decreases.The?¹¹B curve of calcium carbonate is not parallel to the calculated?¹¹B₄curve of artificial magnesium free brine,but slightly deviates with the increase of pH value.The?¹¹B value of calcium carbonate is about1.1‰?3.3‰higher than the calculated?¹¹B₄of artificial magnesium free brine.The results show that with the increase of pH value,B?OH?₃is easier to combine with calcium carbonate,which leads to the deviation of?¹¹B value of carbonate from the calculated?¹¹B₄curve.5.It is found that with the evaporation process,the pH value of the Da Sugan lake decreases,the boron content increases,and the?¹¹B value of the sedimentary carbonate also increases,which indicates that the?¹¹B value of the sedimentary carbonate is consistent with the salinity change during the evolution of the salt lake.If the content of boron in water is low,there are only B?OH?₃and B?OH?₄^-species in water,then the pH,boron concentration and salinity can affect of B?OH?₃/B?OH?₄^-ratio.If the boron content in water is higher?more than 290 mg/L?,the amount of polyborate in water can not be ignored.The?¹¹B value range of fine grained carbonate in East Taijinar salt lake section is-14.78‰to-0.21‰,with an average value of-8.33‰.The variation trend of?¹¹B value is consistent with that of salinity index of boron content and Mg/Ca ratio,and there is a certain positive correlation,which indicates that the?¹¹B value of carbonate can also be used as an indicator of salinity to a certain extent.The?¹¹B value of carbonate and other salinity indicators indicate that East Taijinar salt lake experienced four desalination-concentration processes during the two salt forming periods.